The present invention relates to a hydraulic circuit for a hydraulic shovel adapted to be operated with an increased capacity.
A hydraulic shovel is usually utilized for excavation, but in some case, it is used as a crane for hoisting an object, owing to its high working capacity. For example, a wire rope is hung on a hook fixed to a bucket, and a structure such as a steel pipe is hoisted via the wire rope by operating a boom cylinder and an arm cylinder. In such a hoisting operation, an operating force of the boom cylinder and the arm cylinder is increased by operating a switch located near an operating lever. The switch is operatively connected to a solenoid valve, and the solenoid valve is operatively connected to pilot-operated variable relief valves provided in a main circuit and port circuits of the boom cylinder and the arm cylinder. When the switch is turned on to operate the solenoid valve, set pressures of the variable relief valves are changed.
FIG. 7 shows a conventional hydraulic circuit of a hydraulic shovel as mentioned above. Reference numeral 50 designates a hydraulic pump communicated with a main oil passage 51. The main oil passage 51 is provided with a variable relief valve 52 including a pilot portion 53. The pilot portion 53 is communicated through a pilot oil passage 55 to a solenoid valve 54. The solenoid valve 54 is operated by a switch 56 provided on an operating lever 68.
Reference numeral 57 designates a port oil passage communicated with a head side oil chamber of a boom cylinder 58. The port oil passage 57 is provided with a variable relief valve 59 including a pilot portion 60. The pilot portion 60 is communicated through a pilot oil passage 61 to the solenoid valve 54. Similarly, reference numeral 62 designates a port oil passage communicated with a rod side oil chamber of an arm cylinder 63. The port oil passage 62 is provided with a variable relief valve 64 including a pilot portion 65. The pilot portion 65 is communicated through a pilot oil passage 66 to the solenoid valve 54.
In the case of increasing the hoisting capacity of the boom cylinder and the arm cylinder, the switch 56 of the operating lever 68 is depressed. As a result, the solenoid valve 54 is operated to allow pilot oil from a pilot hydraulic pump 67 to flow through the solenoid valve 54 to the pilot portions 53, 60 and 65 of the relief valves 52, 59 and 64. Accordingly, the set pressures of the relief valves 52, 59 and 64 are increased, thereby increasing operating pressures of the boom cylinder 58 and the arm cylinder 63. Thus, the working capacity of the hydraulic shovel as a crane can be increased.
However, as the set pressure of the variable relief valve 52 provided in the main oil passage 51 is increased, hydraulic pressures of any actuators other than the boom cylinder 58 and the arm cylinder 63 are unnecessarily increased, causing adverse affects on any other hydraulic equipment and structures such as a reduction in durability of the hydraulic equipments or breakage of the structures.
Furthermore, in the event that the hydraulic shovel is used as a normal excavator with an increased working capacity, the hydraulic equipment of the actuators such as a bucket cylinder and a traveling motor which are not required to has increased working capacity are adversely affected to cause a reduction in durability. Further, the structures relating to the actuators are also adversely affected to cause breakage thereof.